Reverse
intersystem crossing (RISC) rate of a thermally activated delayed fluorescence
(TADF) molecule is sensitive to the energy alignment of singlet charge-transfer
state (<sup>1</sup>CT), triplet charge-transfer state (<sup>3</sup>CT), and
locally excited triplet state (<sup>3</sup>LE). However, the energy distribution of the charge-transfer states originating
from the conformational distribution of TADF molecules in a solid matrix
inevitably generated during the preparation of a solid sample due to the
rotatable donor-acceptor linkage is rarely considered. Moreover, the
investigation of the energy distribution of the <sup>3</sup>CT state is both
theoretically and experimentally difficult due to the triplet instabilities of
time-dependent density functional (TD-DFT) calculations and difficulties in phosphorescence
measurements, respectively. As a result, the relation between conformational
distribution, configurations of excited state transition orbitals, and excited
state energies/dynamics have not been clearly explained. In this work, we
determined the energy distribution of CT states of the TADF emitter TPSA in
frozen toluene at 77 K by the measurement of time-resolved spectra in the full
time range (1 ns ~ 30 s) of emission including prompt fluorescence, TADF, <sup>3</sup>CT
phosphorescence, and <sup>3</sup>LE phosphorescence. We obtained the energy
band of CT states where <sup>1</sup>CT and <sup>3</sup>CT states are
distributed in the range of 2.85-3.00 eV and 2.64-2.96 eV, respectively. We
tested various global hybrid and long-range corrected functionals for the TD-DFT calculation of <sup>3</sup>CT energy of TPSA and found
that only the M11 functional shows consistent results without triplet instability.
We performed TD-DFT with the M11* functional optimized for robust dihedral
angle scan of <sup>3</sup>CT states without triplet instability and reproduced
the energy band structure obtained from the experiment. Through TD-DFT and
experimental investigations, it is estimated that the dihedral angle of
donor-acceptor (θ<sub>D-A</sub>) and acceptor-linker (θ<sub>A</sub>) of TPSA in
frozen toluene lie within the range of 70°≤θ<sub>D-A</sub>≤90° and 0°≤θ<sub>A</sub>≤30° respectively. Our results show that the dihedral
angle distribution must be considered for further investigation of the
photophysics of TADF molecules and the development of stable and efficient TADF
emitters.
Band-selective dynamics in charge-transfer excited iron carbene complexes
